This invention relates to the soldering of printed circuit boards (pcb) or the like in which the boards are passed through a heater prior to soldering. More specifically this invention relates to a heater having at least two stages and a control system for the heater.
Prior machines of this type have been developed in which some of the process parameters have been controlled automatically but, as far as is known, no prior machine has been developed which is capable of completely automatically processing the boards through the various steps necessary to ensure efficient and reliable soldering. Perhaps one reason for this is that prior machines have not been able to achieve the degree of accuracy necessary in preheating the boards to ensure proper soldering. The following discussion brings out the importance of heating control in soldering and other processes.
A wave soldering machine is a machine for soldering components premounted on a printed circuit board to that board. The board is held near horizontal with components on the top surface and their leads projecting through holes in the board. Sometimes small components (chips) may be cemented to the bottom surface of the board. This board is passed over the solder wave, just skimming its surface. During this passage, strong, electrically conductive solder joints have to be formed between the components and the board. A well soldered board has the following joint characteristics.
1. All exposed copper and/or other metallic solderable surfaces of the board and component leads are well wetted by solder. PA1 2. All joints are or have a continuous metallurgical structure. PA1 3. Solder has flowed through joint interspaces and holes in the pcb, partly by capillary action to make sound joints and nice through connections to the top surface of the board. PA1 4. There is no bridging between component leads, pads or tracks on the pcb causing a false circuit. PA1 5. There are no icicles of frozen solder hanging from the bottom of the board. PA1 6. None of the components have been damaged by the temperature of the process. PA1 7. There is no excessive alloying between the leads, or copper of the board and the solder. PA1 8. There is no inclusions or impurities in the solder.
Quite a number of these qualities are affected by the temperature of the board when it meets the solder wave.
Obviously if the board is cold and is taken through the solder wave quickly, the solder will freeze onto the cold board without flowing into confined or restricted areas, joints, holes and the like, and will not be forming good joints between well wetted parts.
To maintain a reasonable soldering speed and, therefore, production rate of boards per day the boards need heating prior to soldering. The heating has to achieve a balance between damage by thermal shock and high temperature to the board and components, or excessive soldering time required to transfer heat from the solder of the solder pot to the board.
Heating the board prior to soldering also increases the activity of the flux which improves the general solderability.
The temperature of the board is a dynamic parameter, with temperature gradients through the board which vary with time. The temperature gradients will decay with time so that a long gap between board heating and soldering will give a cooler, even temperatured board, whereas heating in the solder wave will give a hot lower surface and cold top surface to the board with, therefore, large thermal gradients.